Metal-Ligand Core-Shell Nanocomposite Catalysts for the Selective Semihydrogenation of Alkynes

Acceleration of the semi-hydrogenation of alkynes over an N-doped porous carbon sphere-confined ultrafine PdCu bimetallic nanoparticle catalyst

Selective hydrogenation of alkynes to obtain alkenes is a key reaction in petrochemical and fine chemical industries. However, the development of stable and highly selective catalysts with uniformly dispersed active sites is still immensely challenging for the semi-hydrogenation of alkynes. In this study, N-doped porous carbon nanospheres (NPCNs) were synthesized by the nanoemulsion self-assembly and subsequently carbonization method. Ultrafine PdCu bimetallic nanoparticles (NPs) were uniformly dispersed and immobilized on NPCNs. The obtained PdCu/NPCNs catalyst exhibited an open framework and abundant active sites originating from ultrafine PdCu NPs. In the semi-hydrogenation of alkynes, the PdCu/NPCNs catalyst exhibited a remarkable performance and stability, outperforming most of the classical catalysts. The excellent performance was related to the introduction of a secondary metal Cu, which can regulate the electronic state of Pd active sites to further enhance the hydrogenation activity and selectivity. Hence, the facile approach reported herein may be useful for constructing highly dispersed bimetallic NP-based catalysts for selective hydrogenation of alkynes in the petrochemical industry.

Regulating Interfacial Water Structure by Tensile Strain to Boost Electrochemical Semi-hydrogenation of Alkynes

Electrochemical semi-hydrogenation (ECSH) of alkynes to produce alkenes is an ideal alternative to traditional thermal semi-hydrogenation (TSH), and yet is limited by low conversion yield and product selectivity. Here, we...

Dynamic Modification of Palladium Catalysts with Chain Alkylamines for the Selective Hydrogenation of Alkynes

Selective hydrogenation of alkynes plays a pivotal role in the field of chemical production but still suffers from restrained catalytic activity and low alkene selectivity. Herein, a dynamic modification strategy was utilized by preferentially attaching diethylenetriamine (DETA) to the surface of the support to modify the Pd catalyst. The DETA-modified Pd catalyst demonstrates unprecedented reactivity (14,412 h^-1) and selectivity as high as 94% for the semihydrogenation of 2-methyl-3-butyn-2-ol at 35 °C, presenting a 36-fold higher reactivity than the Lindlar catalyst. Moreover, the yield exceeds 98.2% at full conversion under no solvent and organic adsorbate conditions, indicating the potential applications for industrial production. Systematic studies reveal that flexible DETA serves in a reversible "breathing pattern" for the molecular discrimination by constructing dynamic metal-support interaction (DMSI), enabling selective exclusion of alkenes from the Pd surface. DETA is competent to dynamically adjust the adsorption behaviors of reactants and effectively boost the intrinsic activity of the modified catalyst. Impressively, the DETA-modified Pd catalyst exhibits exceptional stability even after being recycled 20 times. This work sheds light on a novel and applicable method for the rational design of heterogeneous catalysts via DMSI.

CuCl2-catalyzed highly stereoselective and chemoselective reduction of alkynyl amides into α,β-unsaturated amides using silanes as hydrogen donors

A CuH-catalyzed Z-selective partial reduction of alkynyl amides to afford α,β-unsaturated amides using silane as the hydrogen donor is developed. This reaction is carried out under mild conditions and able to accommodate a broad scope of alkynyl amides including those bearing a terminal carbon-carbon double bond or triple bond, affording alkenyl amides with high stereoselectivity and excellent yields.

A Smart Heterogeneous Catalyst for Efficient, Chemo- and Stereoselective Hydrogenation of 3-Hexyn-1-ol

We examine the easy preparation of mono- and bi-metallic heterogeneous catalysts with low Pd and Cu contents on alumina and provide a detailed study of many reaction parameters in the catalyzed selective semihydrogenation of 3-hexyn-1-ol to (Z)-3-hexen-1-ol, a very important fragrance with an herbaceous note. In particular, two different protocols of Pd catalyst preparation, substrate/catalyst molar ratio, the effect of time and temperature, introduction of some additives to the reaction mixture, and the nature of the solvent were investigated. These factors are not independent variables. The results show that it is possible to control the reaction outcome to obtain the target (Z)-alkenol using different experimental conditions. The best result, as an appropriate compromise between conversion and selectivity, may be obtained by working with a very high substrate/catalyst molar ratio (>6000/1), with one type of Pd catalyst, in a short time (about 150 min) at 60 °C.

Catalytic, Transition-Metal-Free Semireduction of Propiolamide Derivatives: Scope and Mechanistic Investigation

We report a transition-metal-free trans-selective semireduction of alkynes with pinacolborane and catalytic potassium tert-butoxide. A variety of 3-substituted primary and secondary propiolamides, including an analog of FK866, a potent nicotinamide mononucleotide adenyltransferase (NMNAT) inhibitor, are reduced to the corresponding (E)-3-substituted acrylamide derivatives in up to 99% yield with >99:1 E/Z selectivity. Mechanistic studies suggest that an activated Lewis acid-base complex transfers a hydride to the α-carbon followed by rapid protonation in a trans fashion.

Practical Synthesis and Field Application of the Synthetic Sex Pheromone of Rice Stem Borer, Chilo suppressalis (Lepidoptera: Pyralidae)

Rice stem borer, Chilo suppressalis, is a common and major serious pest of rice, maize, and wheat crops across Asia, Europe, and Oceania countries. Its sex pheromone consists of three analogously compounds, i.e., (Z)-hexadec-11-enal (1), (Z)-octadec-13-enal (2), and (Z)-hexadec-9-enal (3), as long-chain aliphatic internal cis-alkenyl aldehydes. In order to perform an economic and widespread pest control management of rice stem borer, a versatile and efficient synthetic strategy is required. A versatile and efficient synthesis using a common synthetic route for cis-alkenals with high overall yields is described. Commercially available inexpensive aliphatic diols were chosen as starting materials. Two key steps were employed to synthesize the long-chain aliphatic internal cis-alkenes in excellent yields, including the alkylation of terminal alkynes without the utilization of a highly polar aprotic cosolvent and the versatile cis-selective semihydrogenation for the reduction of internal alkynes with excellent stereoselectivity. The results of field tests showed that the synthetic sex pheromone blend was highly effective for the capture of rice stem borer.

Aqueous phase semihydrogenation of alkynes over Ni–Fe bimetallic catalysts

Bimetallic Ni–Fe catalysts (Ni/Fe, 1 : 1, 1 : 3, and 3 : 1) are synthesized and explored for their catalytic activity in semihydrogenation of internal alkynes using H₂ gas in water–ethanol solution.

Engineering of Surface Environment of Pd Nanoparticle Catalysts on Carbon Support with Pyrene-Thiol Ligands for Semihydrogenation of Alkynes

A new type of pyrene-thiol derivative-modified Pd nanoparticle (NP) catalyst on a carbon black support for the efficient semihydrogenation of alkynes to alkenes is reported herein. Colloidal Pd NPs surrounded by pyrene-thiol modifiers were prepared using the two-phase Brust method followed by impregnation of carbon black materials. Based on the structural characterization of the prepared catalyst (PyC₁₂S-Pd/VC) by NMR, UV-vis, FT-IR, TEM, HAADF-STEM, Pd K-edge XAFS, XRD, N₂ adsorption, and XPS, we show that highly dispersed Pd NPs are immobilized on the catalysts via π-π interaction between pyrene groups bound to the Pd NPs and carbon black supports. PyC₁₂S-Pd/VC efficiently catalyzes the alkyne semihydrogenation reaction while maintaining high alkene selectivity; an alkene selectivity of 94% is attained at 98% conversion after 5 h of reaction, and the selectivity was retained around 80% in 10 h of reaction. This performance is superior to that of a catalyst without pyrene groups and that of a commercial Lindlar catalyst. The steric hindrance of pyrene groups restricts access of the substrates to Pd NP surfaces, suppressing the unfavorable overhydrogenation of alkenes to alkanes, which is revealed by the solvent and substrate dependency on the catalytic performance and a DFT calculation study. Furthermore, the high selectivity and stability of PyC₁₂S-Pd/VC are caused by the strong interaction between pyrene groups and carbon supports, which prevents the separation of pyrene modifiers and the leaching or sintering of Pd NPs during the catalytic reaction. It is demonstrated that the combination of Pd NPs, pyrene-thiol modifiers, and carbon supports offers high activity, alkene selectivity, and stability in the semihydrogenation reaction.

Synthesis of Z-Enamides through Heterogeneous Gold-Catalyzed Stereoselective Hydrogenation of Ynamides

A facile synthesis of Z-enamides via heterogeneous Au/TiO₂ catalyzed stereoselective hydrogenation of ynamides has been developed. Easy to handle and inexpensive ammonium formate was used as the hydrogen source, and Z-enamides were formed in a highly stereoselective manner. The commercially available gold nanoparticle catalyst could be recycled multiple times without a significant loss of activity.

Metal Nanoparticles Supported on Biomass-Derived Hierarchical Porous Heteroatom-Doped Carbon from Bamboo Shoots: Design, Synthesis and Applications

Heteroatom-doped porous carbon derived from biomass have recently received increasing attention due to their unique properties such as high electrical conductivity, large specific surface area, high porosity, and easy availability, which are appealing materials for versatile applications in catalysis, energy, separation and adsorption, and life sciences as well. On the basis of our previous work in this field, we summarized in this account our recent progress on design, synthesis of metal (e. g., Pd, Co) nanoparticles supported heteroatom-doped hierarchical porous carbon material derived from bamboo shoots and their applications for important organic transformations, including chemoselective semihydrogenation of alkynes, hydrosilylation of alkynes, cascade synthesis of benzofurans from terminal alkynes and iodophenols, selective hydrogenation of functionalized nitroarenes to form anilines, imines, and formamides. Finally, the current state and future challenges in this field are discussed. We hope this account could shed light on the rational design of novel non-noble metal based heterogeneous catalysts derived from biomass for efficient and sustainable organic transformations.

Selective Hydrogenations and Dechlorinations in Water Mediated by Anionic Surfactant-Stabilized Pd Nanoparticles

We report a facile, inexpensive, and green method for the preparation of Pd nanoparticles in aqueous medium stabilized by anionic sulfonated surfactants sodium 1-dodecanesulfonate 1a, sodium dodecylbenzenesulfonate 1b, dioctyl sulfosuccinate sodium salt 1c, and poly(ethylene glycol) 4-nonylphenyl-3-sulfopropyl ether potassium salt 1d simply obtained by stirring aqueous solutions of Pd(OAc)₂ with the commercial anionic surfactants further treated under hydrogen atmosphere for variable amounts of time. The aqueous Pd nanoparticle solutions were tested in the selective hydrogenation reactions of aryl-alcohols, -aldehydes, and -ketones, leading to complete conversion to the deoxygenated products even in the absence of strong Brønsted acids in the reduction of aromatic aldehydes and ketones, in the controlled semihydrogenation of alkynes leading to alkenes, and in the efficient hydrodechlorination of aromatic substrates. In all cases, the micellar media were crucial for stabilizing the metal nanoparticles, dissolving substrates, steering product selectivity, and enabling recycling. What is interesting is also that a benchmark catalyst like Pd/C can often be surpassed in activity and/or selectivity in the reactions tested by simply switching to the appropriate commercially available surfactant, thereby providing an easy to use, flexible, and practical catalytic system capable of efficiently addressing a variety of synthetically significant hydrogenation reactions.

Borylation of α,β-Unsaturated Acceptors by Chitosan Composite Film Supported Copper Nanoparticles

We describe here the preparation of copper nanoparticles stabilized on a chitosan/poly (vinyl alcohol) composite film. This material could catalyze the borylation of α,β-unsaturated acceptors in aqueous media under mild conditions. The corresponding organoboron compounds as well as their converted β-hydroxyl products were all obtained in good to excellent yields. It is noteworthy that this catalyst of copper nanoparticles can be easily recycled eight times and remained catalytically reactive. This newly developed methodology provides an efficient and sustainable pathway for the synthesis of organoboron compounds and application of copper nanoparticles.

Polyhedral Cu2O to Cu pseudomorphic conversion for stereoselective alkyne semihydrogenation

Cu₂O nanocrystals can be pseudomorphically converted into Cu crystals through ammonia borane reduction, releasing hydrogen for stereoselective semihydrogenation of diphenylacetylene.

Cu₂O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted into Cu crystals of corresponding shapes through reduction by ammonia borane in ethanol at 50 °C or below within 3 min, demonstrating the feasibility of making challenging polyhedral metal particles from metal oxide crystals. Hydrogen gas is also produced from ammonia borane in the process. The obtained Cu crystals have a slightly nanoporous interior. Addition of diphenylacetylene in the formation of Cu rhombic dodecahedra leads to complete stereoselective production of sterically hindered (Z)-stilbene. Semihydrogenation of other alkynes also gives pure (Z)-alkenes. Cu cubes and octahedra also showed considerable (Z)-stilbene selectivity along with minor formation of (E)-stilbene and bibenzyl as compared to CuCl₂ and commercial Cu₂O particles. Mechanistic studies reveal that the low binding affinity of alkenes on the rhombic dodecahedra surfaces leads to high product selectivity. These Cu crystals act as a green and low-cost catalyst for the synthesis of high-purity (Z)-alkenes.

Additive-modulated switchable reaction pathway in the addition of alkynes with organosilanes catalyzed by supported Pd nanoparticles: hydrosilylation versus semihydrogenation

A supported Pd nanoparticles on N,O-doped hierarchical porous carbon enabled additive-modulated reaction pathway for alkyne addition with organosilanes between hydrosilylation and semihydrogenation.

Composition-Dependent Morphology of Bi- and Trimetallic Phosphides: Construction of Amorphous Pd-Cu-Ni-P Nanoparticles as a Selective and Versatile Catalyst

Amorphous materials have been widely researched in heterogeneous catalysis and for next-generation batteries. However, the well-defined production of high-quality (e.g., monodisperse and high surface area) amorphous alloy nanomaterials has rarely been reported. In this work, we investigated the correlations among the composition, morphology, and catalysis of various Pd-M-P nanoparticles (NPs) (M = Cu or Ni), which indicated that less Cu (≤20 atom %) was necessary for the formation of an amorphous morphology. The amorphous Pd-Cu-Ni-P NPs were fabricated with a controllable size and characterized carefully, which show excellent selective catalysis in the semihydrogenation of alkynes, hydrogenation of quinoline, and oxidation of primary alcohols. The uniqueness of the catalytic performance was confirmed by control experiments with monometallic Pd, amorphous Pd-Ni-P NPs, crystalline Pd-Cu-P NPs, and a crystalline counterpart of Pd-Cu-Ni-P catalyst. The catalytic selectivity likely arose from improved Pd-M (M = Cu or Ni) synergistic effects in the amorphous phase and the electron deficiency of Pd. The model reactions proceeded under H₂ or O₂ gas without any additives, bases, or metal oxide supports, and the catalyst could be reused several times. This report is expected to shed light on the design of amorphous alloy nanomaterials as green and inexpensive catalysts for atom-economic and selective reactions.

Selective Semihydrogenation of Alkynes Catalyzed by Pd Nanoparticles Immobilized on Heteroatom-Doped Hierarchical Porous Carbon Derived from Bamboo Shoots

Highly dispersed palladium nanoparticles (Pd NPs) immobilized on heteroatom-doped hierarchical porous carbon supports (N,O-carbon) with large specific surface areas are synthesized by a wet chemical reduction method. The N,O-carbon derived from naturally abundant bamboo shoots is fabricated by a tandem hydrothermal-carbonization process without assistance of any templates, chemical activation reagents, or exogenous N or O sources in a simple and ecofriendly manner. The prepared Pd/N,O-carbon catalyst shows extremely high activity and excellent chemoselectivity for semihydrogenation of a broad range of alkynes to versatile and valuable alkenes under ambient conditions. The catalyst can be readily recovered for successive reuse with negligible loss in activity and selectivity, and is also applicable for practical gram-scale reactions.